The present invention relates in general to mine roof supports and more particularly to roof-contacting structures of such supports which are provided with appliances for forwardly extending the structures. The term "roof-contacting structures" is intended to refer to simple bars or girders or to larger caps, linings or canopies which make contact with the roof of a mine working when the props of the support are extended.
It is known to provide the main roof-contacting structure e.g. a cap or bar, of a mine roof support with an auxiliary lining or structure which is pivotably connected thereto. A piston and cylinder unit can then be used to pivot the auxiliary roof-contacting structure about the main structure to bring the auxiliary structure into contact with the roof of the pertinent mine working. Other types of advanceable or forwardly extendible appliances cause the auxiliary structure to slide in or out relative to the main structure.
German patent specification No. 1230745 describes an appliance of the aforementioned kind wherein the piston and cylinder unit used to pivot the auxiliary bar is protected from the forces exerted by the roof on the auxiliary bar. The piston and cylinder unit is pivotably connected to an intermediate guide piece which is slidably mounted to the main bar and which connects to the auxiliary bar via a short pivotal strap. This known arrangement is apt to suffer from considerable wear, especially the slidable mounting of the intermediate guide piece, and is relatively costly to manufacture. In addition, the auxiliary bar can only be pivoted through a relatively small angle, e.g. 15.degree.. In the event of excessive roof pressure, the auxiliary bar of this known arrangement is prone to suffer damage together with the associated guide components even if the piston and cylinder unit is itself protected by the self-locking action of the guide components. The self-locking action of the guide components also means that the portion of the roof in contact with the auxiliary bar is not always adequately supported since the piston and cylinder unit forms no part of the support.
U.S. Pat. No. 4,347,021 assigned the same assignee as the present application is concerned with an improved pivoting linkage composed of several levers coupled between the auxiliary and main bars and the piston and cylinder units. In this arrangement, the auxiliary bar can be swung through a considerable angle in the order of 180.degree. between its operative and stowed positions.
A general object of this invention is to provide an improved form of extendible roof-contacting structure for a mine roof support.